Polypropylene is an inexpensive thermoplastic polymer employed in a wide variety of applications, the articles of which include, for example, films, fibers, such as spunbonded and melt blown fibers, fabrics, such as nonwoven fabrics, and molded articles. The selection of polypropylene for any one particular application depends, in part, on the physical and mechanical properties of the polypropylene polymer candidate as well as the article fabrication mode or manufacturing process. Examples of physical properties include density, molecular weight, molecular weight distribution, melting temperature and crystallization temperature. Examples of mechanical properties include heat distortion temperature (HDT) and Flexural Modulus. Examples of factors relevant to the processing environment include the melt flow rate (MFR), cycle time, bubble stability, sag resistance, melt strength and shear/elongational viscosity.
In some instances articles formed from polypropylene, for example, via an injection molding process, may require a high degree of structural rigidity. This structural rigidity may be directly correlated with the value of modulus (e.g. flexural modulus), such that to achieve high structural rigidity in a molded article, polymers exhibiting high modulus values are desirable. Additionally, for such articles to be economically manufactured, the fabrication mode must be capable of producing the article at a selected rate, also referred to as “cycle time”. The cycle time for injection molding may generally be described as the duration from the introduction of molten polymer into the mold to the release of the molded article from the mold. The cycle time is a function of the viscosity of the molten polymer. Cycle time also relates to the crystallization temperature of the polymer. Generally, the crystallization temperature is the pivotal temperature at which the molten liquid polymer hardens. This hardening is due, in part, to the formation of crystalline structures within the polymer. It follows that as the molten polymer cools in the mold, molten polymers having higher crystallization temperatures will form crystalline structures sooner than polymers having lower crystallization temperatures. As such, shorter cycle times may be achieved by using polymers with higher crystallization temperatures. It will be understood from this that many variables may be relevant and require consideration before selecting a polymer for a particular application.
As the criteria for polymer applications and articles formed therefrom continue to evolve, there remains a need to continually modify and improve the physical, mechanical and rheological properties of polymers, and in particular polypropylene polymers, to meet these evolving criteria.